Wet press felts are used in the press section of papermaking machines to transport and dewater an aqueous paper web which is being made into a desired paper product. The dewatering process conventionally entails transporting the aqueous web through a series of pressure nips.
It is well known in the art to employ press felts comprised of a base fabric with fibrous batt material needled thereto. The fibrous batt generally provides a smooth paper carrying surface and a resilient structure to assist in the dewatering of the aqueous paper web. Any irregularities in the wet felt can result in undesirable marking of the aqueous paper web since it is highly deformable at this stage of the papermaking process.
A wide variety of base fabric constructions are known in the art. For example, the base fabric may be comprised of a single layer or multiple layers of machine direction yarns, having a relatively large diameter to provide machine direction strength, interwoven with smaller diameter cross machine direction yarns.
Even though the base fabric is covered with batt material, the yarns which predominate the paper carrying side of the base fabric can cause marking on the aqueous paper web. In particular, the yarns which predominate on the paper carrying side of the base fabric can lead to problems with marking of the aqueous sheet. Such problems may not be initially apparent. However, they can occur after the fabric has been used on papermaking equipment since the batt materials becomes matted and/or compacted through use. This tends to augment the effect of the base fabric on the aqueous web.
Additionally, base fabrics have been designed to enhance the overall resiliency of the press felt to assist in the dewatering process. U.S. Pat. No. 4,537,816, assigned to the assignee of the present invention, discloses a wet felt having a selectively configured base fabric structure which is designed with void areas to enhance the resiliency and dewatering ability of the wet felt.
Another example of a base fabric designed to enhance the overall resiliency of press felt is disclosed in U.S. Pat. No. 4,883,097, assigned to the assignee of the present invention, wherein a compressible, resilient knit yarn is incorporated into the weave of the base fabric structure.
Additionally, U.S. Pat. No. 4,414,263, discloses the use of flat cross machine direction (CMD) yarns to decrease the prominence of CMD yarn knuckles in the base fabric of a wet press felt to avoid problems with marking.
Hollow synthetic monofilament yarns are known in the papermaking arts. Such yarns are disclosed in U.S. Pat. No. 4,251,588, for the manufacturer of a dryer fabric having improved dimensional stability. As noted therein, such conventional, hollow monofilament yarns have a core void of about 3% to 15% of their cross-sectional area to avoid flattening.
In the context of a dryer fabric application, applicant's assignee has experimented with using hollow monofilament yarns having a core void in excess of 30%. In such application, the wall thickness of the yarns is designed to become flattened in the weave to reduce the permeability of the dryer fabric.
Other uses of hollow yarns are known in the art. For example, U.S. Pat. No. 2,132,252, discloses the use of hollows metal wires in the creation of a forming fabric. However, such forming fabrics are not subjected to pressure nips and accordingly do not demand the resiliency of a press fabric. Additionally, U.S. Pat. No. 4,569,883, employs hollow fibers in the composition of spun yarns and/or to comprise the fibrous batt material for a wet press felt.